What Happens if I Workout Without Eating?

Food intake and physical activity for the average person is a debatable topic. Some research indicates that there are benefits to exercising on an empty stomach, while other evidence suggests that the negative effects may outweigh the benefits. Fat burn may increase if you workout without eating, but you also risk using muscle for fuel and limiting your exercise duration due to fatigue. Understand the facts, then consult your doctor or a professional regarding the best course of action for your condition.

woman working out in gym (Image: Ibrakovic/iStock/Getty Images)

The Early Bird Gets the Burn

A study published in the "Journal of Physiology" in 2010 found that working out without eating may increase fat burn. Researchers took 28 healthy adults and had them eat a diet with 50 percent more fat and 30 percent more calories than their normal diets. Some of the men abstained from exercise, while the others were instructed to exercise four times per week in the mornings. Some of the exercisers ate breakfast before exercising and others did not. At the end of the study, the participants skipping breakfast before exercise gained less weight and experienced improvements in insulin sensitivity than those who ate before exercising.

Intensity is Key

Exercising on an empty stomach may be most effective when doing steady-state cardio. However, high-intensity exercises like heavy resistance training rely primarily on glucose for muscle contraction. If glucose stores are low after several hours of fasting, your body may break down your lean muscle mass for fuel, defeating the purpose of your workout. Your training status, stores of intramuscular fat and the capacity of your muscles to store glucose in the form of glycogen are primary determinants of energy pathways during intense exercise.

Brain Drain

Exercising without eating first can lead to low blood glucose, which can interfere with your brain function. When you eat, glucose levels in your circulating blood rise and are available to travel to your muscle cells. Glucose is also available as glycogen stored in your muscles. When you work out before eating, glycogen and blood glucose can become quickly depleted, causing hypoglycemia. Because your brain runs exclusively on glucose, low levels may cause light-headedness, nausea, muscle fatigue and poor exercise performance. Regular ongoing training enhances your muscles' capacity to store glycogen.

Plan for Performance

Timing and quantity of food can influence your exercise performance. For optimal exercise capacity follow these guidelines: If you eat a large meal, wait three to four hours before exercising. For a small meal, eat two to three hours pre-exercise. A snack is your best option, eaten an hour before exercise. Try consuming a piece of fruit, yogurt or granola before exercise to maximize blood glucose and calorie burn. Eating a snack combining protein and carbohydrates immediately after exercise will replenish muscle glycogen stores, setting you up for your next workout. Adequate hydration and replacement of electrolytes are also critical for peak exercise performance.

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